Low Excitation Energy Region Research Articles

Constraining the solar neutrino survival probability curve by using Li6 , Li7 , C12 , O18 , F19 , and Ca42 nuclear targets

Precise measurement of the survival probability ${P}_{ee}$ of solar electron neutrino (${\ensuremath{\nu}}_{e}$) as a function of its energy [${P}_{ee}({E}_{\ensuremath{\nu}e})$] is one of the key issues in neutrino physics. Current ${P}_{ee}$ data, due to their limited accuracy, still allow for nonstandard interactions (NSIs) to be alternatives to the standard one, which is based on the MSW-LMA prediction. In order to determine ${P}_{ee}$ values at several values of ${E}_{\ensuremath{\nu}e}$ with higher accuracy, we propose to use several target nuclei with different threshold energies for ${\ensuremath{\nu}}_{e}$ detection. We examined charged-current (CC) responses of various nuclei seeking the ones: (a) having large and concentrated Gamow-Teller (GT) transition strength in the low excitation-energy region, and (b) having appropriate and a variety of reaction $Q$ values, i.e., $\ensuremath{\approx}1$ to 17 MeV, in the (${\ensuremath{\nu}}_{e},{e}^{\ensuremath{-}}$) reaction. As a result, we found that systematic solar ${\ensuremath{\nu}}_{e}$ measurements with target nuclei $^{6}\mathrm{Li}$, $^{7}\mathrm{Li}$, $^{12}\mathrm{C}$, $^{18}\mathrm{O}$, $^{19}\mathrm{F}$, and $^{42}\mathrm{Ca}$ can put strong constraints on the ${P}_{ee}({E}_{\ensuremath{\nu}e})$ curve and thus on these NSI models. In addition, we notice that three of these nuclei, $^{6}\mathrm{Li}$, $^{7}\mathrm{Li}$, and $^{12}\mathrm{C}$, have large and concentrated neutral-current (NC) responses with detection threshold-energies of 3.56, 0.48, and 15.11 MeV, respectively. Note that the NC measurement is flavor independent. Thus, the measured results should represent the original strength of ${\ensuremath{\nu}}_{e}$ from the Sun.

Structure of Mg31 : Shape coexistence revealed by β−γ spectroscopy with spin-polarized Na31

The level structure of $^{31}\mathrm{Mg}$, which is located close to the region of the $N$ = 20 ``island of inversion'' in the nuclear chart, has been studied by $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy with spin-polarized $^{31}\mathrm{Na}$. In $^{31}\mathrm{Mg}$, shape coexistence is expected as a result of subtle competition between the spherical mean field and the nuclear correlation which favors deformed configurations. In the present work, our unique method utilizing the anisotropic $\ensuremath{\beta}$ decay of spin-polarized $^{31}\mathrm{Na}$ enables us to firmly assign the spins of all positive-parity excited levels in $^{31}\mathrm{Mg}$ below the neutron separation energy at 2.3 MeV. Furthermore, by constructing a very detailed decay scheme, including two newly found levels, the spins of negative-parity levels are restricted. The examination of the spectroscopic information shows that the deformed rotational bands with ${K}^{\ensuremath{\pi}}=1/{2}^{+}$ and $1/{2}^{\ensuremath{-}}$, which have very similar structures to those observed in a higher excitation energy region of $^{25}\mathrm{Mg}$, appear as the ground-state and low-lying bands, respectively, in $^{31}\mathrm{Mg}$. The experimental levels of $^{31}\mathrm{Mg}$ are compared, on the level-by-level basis, with two types of theoretical calculations. These are, first, the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM) and, second, the shell model with the EEdf1 interaction, which is microscopically derived from chiral effective field theory. It is understood that 8 levels among the experimental 11 levels are the members of four types of largely deformed rotational bands and 2 levels are of spherical nature. The $1/{2}^{+}$ 2.244-MeV level is successfully reproduced by the shell-model calculation with a dominant 4p4h configuration. The present work clearly demonstrates that various structures coexist in a low excitation energy region of $^{31}\mathrm{Mg}$.

Bose-Einstein condensation of α clusters and new soft mode in C12 – Fe52 4N nuclei in a field-theoretical superfluid cluster model

Bose-Einstein condensation of alpha clusters in light and medium-heavy nuclei is studied in the frame of the field theoretical superfluid cluster model. The order parameter of the phase transition from the Wigner phase to the Nambu-Goldstone phase is a superfluid amplitude, square of the moduli of which is the superfluid density distribution. The zero mode operators due to the spontaneous symmetry breaking of the global phase in the finite number of alpha clusters are rigorously treated. The theory is systematically applied to N alpha nuclei from12C-52Fe at various condensation rates. In 12C it is found that the energy levels of the gas-like well-developed alpha cluster states above the Hoyle state are reproduced well in agreement with experiment for realistic condensation rates of alpha clusters. The electric E2 and E0 transitions are calculated and found to be sensitive to the condensation rates. The profound raison d'etre of the alpha cluster gas-like states above the Hoyle state, whose structure has been interpreted geometrically in the nuclear models without the order parameter such as the cluster models or ab initio calculations, is revealed. It is found that in addition to the Bogoliubov-de Gennes vibrational mode states collective states of the zero mode operators appear systematically at low excitation energies from the N alpha threshold energy. These collective states, new-type soft modes in nuclei due to the Bose-Einstein condensation of the alpha clusters, emerge systematically in light and medium-heavy mass regions and are also located at high excitation energies from the ground state in contrast to the traditional concept of soft mode in the low excitation energy region.

Shape coexistence in the N = 19 neutron-rich nucleus 31 Mg explored by β – γ spectroscopy of spin-polarized 31 Na

Abstract The structure of excited states in the neutron-rich nucleus 31 Mg, which is in the region of the “island of inversion” associated with the neutron magic number N = 20 , is studied by β – γ spectroscopy of spin-polarized 31 Na. Among the 31 Mg levels below the one neutron separation energy of 2.3 MeV, the spin values of all five positive-parity levels are unambiguously determined by observing the anisotropic β decay. Two rotational bands with K π = 1 / 2 + and 1 / 2 − are proposed based on the spins and energies of the levels. Comparison on a level-by-level basis is performed between the experimental results and theoretical calculations by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM). It is found that various nuclear structures coexist in the low excitation energy region in 31 Mg.

Search for new resonant states in10C and11C and their impact on the cosmological lithium problem

The observed primordial 7Li abundance in metal-poor halo stars is found to be lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of approximately three. Some recent works suggested the possibility that this discrepancy originates from missing resonant reactions which would destroy the 7Be, parent of 7Li. The most promising candidate resonances which were found include a possibly missed 1- or 2- narrow state around 15 MeV in the compound nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation energy region of 10C and the low excitation energy region in 11C via the reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution for the 7Li problem. Concerning 11C results, the data show no new resonances in the excitation energy region of interest and this excludes 7Be+4He reaction channel as an explanation for the 7Li deficit.

Spectroscopic Measurement in 9He and 12Be

Spectroscopy for neutron-rich nuclei 9He and 12Be has been performed by means of heavy-ion double charge exchange (HIDCX) (18O, 18Ne) reaction on stable target nuclei 9Be and 12C, respectively. Several clear peaks in 12Be at low excitation energy region even above neutron separation energy have been observed in one-shot measurement, whereas no prominent signals of 9He have been observed owing to the small cross section. It is shown that the HIDCX reaction is a new powerful spectroscopic tool for study of unstable neutron-rich nuclei.

Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV-UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV-UV hole-burning spectroscopy. The 0-0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm(-1), respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm(-1)) between the two 0-0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.

β-decay in neutron-deficient Hg, Pb, and Po isotopes

The effect of nuclear deformation on the energy distributions of the Gamow-Teller strength is studied in neutron-deficient Hg, Pb, and Po even isotopes. The theoretical framework is based on a self-consistent deformed Skyrme Hartree-Fock mean field with pairing correlations between like nucleons in BCS approximation and residual spin-isospin interactions treated in the proton-neutron quasiparticle random phase approximation. After a systematic study of the Gamow-Teller strength distributions in the low excitation energy region, relevant for beta-decay, we have identified the best candidates to look for deformation signatures in their beta-decay patterns. beta+ half-lives and total Gamow-Teller strengths B(GT+) and B(GT-) are analyzed as well.

Electronic states and transitions in C60 and C70 fullerenes.

A review of the most relevant aspects of fullerene electronic structure and spectroscopy is presented. Experimental data and their interpretation based on computational results are discussed both for fullerene C60 and C70, with particular attention to the properties of the isolated molecule. Concerning singlet state spectroscopy, it is shown that because of its high symmetry, only dipole-forbidden electronic states are found in the low excitation energy region of C60. Conversely, the lowering of symmetry in C70 leads to several complications in its electronic structure and spectroscopy, due to the presence of weakly allowed transitions in the low excitation energy region. A slightly less congested distribution of low lying excited states characterizes the triplet manifold of the fullerenes. It is concluded that while C60 is important in aiding understanding of the main features in electronic spectroscopy of fullerenes, such as the presence of strong absorptions in the high energy range, its spectra are deeply Influenced by its high symmetry and are very peculiar. On the other hand, C70, with its lower symmetry and more complex spectra, represents a more realistic model for the intricate details of the electronic structure and electronic spectroscopy of larger and smaller fullerenes and their derivatives, which are generally characterized by lower symmetry compared to C60.

Low-energy dipole modes in unstable nuclei

Enhancement of electric dipole (E1) strength at low energy is investigated in light neutron and proton drip-line nuclei with halo or skin by large scale shell model calculations. Large E1 strength are found in low excitation energy region below 5 MeV in 11Li, 12Be and 13O. Both the effects of extended halo or skin wave functions and the coherence in the transition amplitudes are important to enhance the E1 strength. The particle (hole)- vibration coupling model is shown to explain the splitting of the low energy E1 strength in 11Li and 13O. Melting of the shell magicity at N=8 and Z=8 is pointed out. Pigmy resonances in oxygen isotopes are also studied. The pigmy strength below E x = 15 MeV are shown to have about 10 % of the Thomas- Reiche-Kuhn (TRK) sum rule and more than 40 % of the cluster sum rule.

Shell-model study of electric dipole and spin-dipole strengths in 11Li

We have studied electric dipole (E1) and spin-dipole strength distribution in 11Li by shell model calculations with halo effect. Two peaks in the E1 strength are found in the low excitation energy region below E x =4 MeV, which have almost the same energies as observed E1 peaks in the Coulomb breakup reaction of 11Li. The calculated E1 strength up to E x =4 MeV exhaust about 4% of the Thomas–Reiche–Kuhn energy-weighted sum rule value. We found also pigmy and giant peaks in spin-dipole strengths of 11Li. Possible existence of double soft dipole mode and giant resonance built on the soft dipole states are investigated.

Nuclear spectroscopy, nuclear clustering and exotic shapes

Nuclear clustering has been a subject of intense study since the advent of heavy ion accelerators. Looking back more than 30 years, we are able today to see the connection between resonances in heavy ion reactions and extreme states in nuclei. In particular, excited states close to the decay thresholds into substructures show pronounced cluster structures. The ever repeated question about the possibility to describe such cluster phenomena in an extended (to a very large basis) and deformed shell model, has recently been revived in view of improved computational facilities. The new experimental facilities and the powerful new detector arrays for γ-spectroscopy and for charged particles have increased the sensitivity to study clustering phenomena by orders of magnitude. Today we are able to study nuclear structure at the limits of the mean field concept. With the advent of radioactive beams and the study of nuclei, which are weakly bound in their ground states, the question of the limits of mean field concepts has reached the low excitation energy region of nuclei.

Spectral anomalies of even-mass Hg isotopes.

The $^{184--200}\mathrm{Hg}$ isotopes are studied in the interacting-boson-approximation-plus-fermion-pair model. In general, energy levels up to I=22 can be reproduced quite well. In the low excitation energy region both the shape evolution and the (${8}^{+}$,${10}^{+}$,${12}^{+}$) triplet structure can be reproduced. The (${8}^{+}$,${10}^{+}$,${12}^{+}$) triplet structure can be understood as the saturation of isospin 1 fermion-pair interaction at the high spin end. The termination of triplet structure at I=12 strongly supports the (${\mathit{i}}_{13/2}$${)}^{2}$ band crossing. Striking agreements between the observed energy gaps of (${14}_{1}^{+}$,${12}_{1}^{+}$) and (${2}_{1}^{+}$,${0}_{1}^{+}$) states are considered as an indication of weak coupling between the boson core and the fermion pair. The B(E2,I\ensuremath{\rightarrow}I-2) are calculated and the spin dependence of the B(E2) values can be reproduced qualitatively. However, the detailed mass dependence is not correctly reproduced.

Spin Excitations in Nuclei

We discuss spin excitations in 3 regions. The low excitation energy region dominated by giant resonances of low multipolarity. The spin response in the quasielastic region and finally the Δ-excitation. We are especially interested in correlations in the spin longitudinal channel.

Reflection electron energy loss spectroscopy (REELS) of conductive polymers obtained by keV bombardment

Reflection electron energy loss (REELS) is used to study the modifications in the electronic structure of polystyrene. poly-2-vynilpyridine, polyethersulphone and polyimide bombarded with Ar + at 150 keV at fluences between 5 × 10 12 and 1 × 10 16 ions/cm 2. Both the bulk plasmon energy and the low-excitation energy region of the REELS spectra are strongly affected by ion irradiation. The ion beam induced spectral features for fluences up to 1 × 10 15 ions/cm 2 are close to those of hydrogenated amorphous carbons. Starting at a fluence of 5 × 10 15 ions/cm 2, Poly-2-vynilpyridine, polyethersulphone and polyimide show a drastic drop in resistivity up to value of the order of 10 −1 − 10 −2 Ω cm. The REELS spectra of these conducting samples are characterized by a peculiar step-like structure at very low excitation energy, attributable to the formation of conduction states in the pristine band-gap.

The isoscalar strength distribution in 24, 26Mg, 28Si and 40Ca obtained from inelastic alpha scattering at 120 Mev

The inelastic α-scattering reaction at E α = 120 MeV with an energy resolution of 90–150 keV has been used to investigate isoscalar strength distributions in 24, 26Mg, 28Si and 40Ca. For 24, 26Mg and 28Si the E2 strength between E x = 14 and 27 MeV is strongly fragmented. In 40Ca the E2 strength is mainly concentrated near E x ~ 65 A 1 3 MeV, although here the onset of fragmentation can be observed. The sum rule strength for the different multipolarities was obtained by applying for each nucleus an L-dependent normalization procedure. In this way we observed in total in 24, 26Mg, 28Si and 40Ca for excitation energies up to 27 MeV an amount of (61 +8 −6), (50 +9 −8), (38 +8 −6) and (94 ± 14)%, respectively, of the isoscalar E2 energy weighted sum rule (EWSR) of which (36 +7 −5), (28 +8 −7), (24 +7 −5) and (74 ± 12)% was found between E x = 14 and 27 MeV. In addition isoscalar E0, E3 and E4 strength was observed in this excitation energy region. A detailed comparison has been made between the isoscalar quadrupole strength distribution observed in the 24, 26Mg(α, α') reaction and the E2 strength excitation function obtained from radiative α-capture measurements. In the low excitation energy region coupled channel effects have been observed, especially for the excitation of the 3 + states. Moreover, a considerable percentage of the 1ħω isoscalar dipole and octupole strength has been observed for excitations below 14 MeV.